Manufacture of pigment grade titanium dioxide requires several processing steps. Generation of a raw or crude titanium dioxide pigment, via calcination in a sulfate process or oxidation in a chloride process, is an intermediate step in the pigment manufacturing process. The raw pigment is then isolated from undesirable salts and conventionally milled as a slurry with various surface treatments to yield a pigment having a desired particle size distribution. Further, certain manufacturing processes benefit from the use of a slurry of a finished titanium dioxide. However, the slurry generated during the manufacturing process contains an excess of water leading to increased shipping costs. Therefore, methods for dewatering titanium dioxide slurries have been developed.
One currently available system for dewatering slurries is the VSEP Filtration System available from New Logic Research Inc. (V⋄SEP{dot over ( )} is a registered trademark of New Logic Research, Inc.). The VSEP process utilizes a filtration system (the “VSEP filtration system”) containing a filter stack consisting of a series of disks or trays and diverters. Each filter disk has outer membranes with pore sizes generally no larger than 0.1μ and normally a drain cloth positioned between the outer membranes and the disk. The diverters determine the flow pattern through the filter stack. A typical VSEP filtration system is schematically depicted in FIG. 1 and a typical filter stack is depicted in FIG. 2. VSEP filtration systems are explained in greater detail, for example, in U.S. Pat. Nos. 4,872,988 and 4,952,317, incorporated herein by reference.
The VSEP filtration system concentrates the slurry of titanium dioxide by removing water from the slurry as it passes over the filter disks and diverters. As the specific gravity of the slurry of titanium dioxide increases, particle build up on the filter disks and diverters will occur unless the shear rate at the membrane surface is sufficient to remove the particles. To generate the necessary shear rate, the VSEP filtration system typically oscillates the trays at an amplitude of movement of about 1.9 cm (0.75 inches). This oscillation produces a shear rate at the membrane-liquid interface of about 150,000 inverse seconds. The VSEP system produces a clear water stream, referred to as permeate, and a slurry of titanium dioxide, referred to as a concentrate.
When performing within the manufacturer's specification, a standard high solids VSEP filter stack lasts between 1 and 45 days and has an average life span of 18 days. However, if operated without tray oscillation, the VSEP filter stack has a life span of only one to two hours. When compared to other filtration systems, such as cross-flow filtration, the VSEP filtration system provides superior performance. However, it would be desirable to increase the useful life span of the filter stacks used in the system and to reduce overall failures of the filter stack.